1. Field of the Invention
The present invention relates to an integrated GPS/IMU attitude sensing apparatus for determining the attitude of a mobile unit by integrating attitude data derived from the global positioning system (GPS) and attitude data derived from an inertial measurement unit (IMU). The GPS-derived attitude and the IMU-derived attitude are hereinafter referred to as the GPS attitude and the IMU attitude, respectively. More particularly, the invention is concerned with an integrated GPS/IMU attitude sensing apparatus designed to reliably estimate an alignment angle for correcting misalignment between an antenna coordinate system and an IMU coordinate system.
2. Description of the Related Art
A GPS attitude sensing system is a known example of a system for determining the heading and attitude of a mobile unit. The conventional GPS attitude sensing system uses at least three GPS antennas which are installed on a rigid mobile unit and are not arranged in a line. The system receives radio signals from GPS satellites through the individual GPS antennas of which positions are known in a 3-axis Cartesian coordinate system, and observes carrier phase differences between the radio signals received by the individual antennas. The system then establishes an antenna coordinate system by calculating relative positions of the GPS antennas from observables of the carrier phase differences and determines the heading and attitude of the mobile unit in a specific reference coordinate system (defined by users).
The conventional GPS attitude sensing system of this kind can determine the attitude of the mobile unit by receiving a radio signal from a GPS satellite. The GPS attitude sensing system however has a problem that, if the radio signal from the GPS satellite is interrupted or a cycle slip in carrier phase observation occurs, it becomes impossible to observe carrier phase differences, resulting in an inability to determine the attitude of the mobile unit.
One known approach to the solution of this problem is GPS/IMU integration technology, in which an inertial attitude sensing system observes motion of a mobile unit by use of inertia sensors (IMUs), such as angular velocity sensors or acceleration sensors, and the amount of rotation of the coordinate system, or an alignment angle for correcting misalignment between the attitude of the mobile unit obtained from inertial observations and the attitude of the mobile unit obtained by the GPS attitude sensing system, is estimated to determine the correct attitude of the mobile unit.
This conventional integration approach integrates attitude observations obtained by the inertial attitude sensing system and the GPS attitude sensing system to give high-precision attitude measurements in a stable fashion. To achieve this, the conventional integration approach involves a process of estimating an alignment angle between the attitude of the mobile unit represented in an inertial sensor coordinate system (IMU coordinate system) which is obtained by the inertia sensors mounted on the individual axes of a 3-axis Cartesian coordinate system and the attitude of the mobile unit represented in an antenna coordinate system which is obtained by the GPS attitude sensing system.
Using this conventional approach, it is possible to observe the motion of the mobile unit by the inertia sensors and uninterruptedly outputs data on the attitude of the mobile unit even when the radio signals from the GPS satellites are interrupted, because attitude observations, if any interrupted due to a loss of the radio signals, can be interpolated by the attitude obtained by the inertia sensors.
The aforementioned conventional GPS/IMU integration approach still has a problem to be solved, however, which is explained in the following.
In the conventional GPS/IMU integration approach, it is necessary to determine the amount of coordinate system rotation, that is, an inherent alignment angle for correcting misalignment between the antenna coordinate system and the IMU coordinate system, at the time of installation of the GPS antennas and the inertia sensors.
Conventionally, estimation of alignment angles is made by one of the following methods.
A first method of alignment angle estimation is such that multiple GPS antennas are installed while visually ensuring, for instance, that one reference direction (axis) of the IMU coordinate system of an inertial attitude sensing system including multiple inertia sensors matches one reference direction (axis) of the antenna coordinate system defined by the multiple GPS antennas. Then, disregarding misalignment which may occur between the two coordinate systems at installation, it is assumed that the two coordinate systems have been exactly matched.
In this first method, misalignment of a few degree could frequently occur between the two coordinate systems, so that attitude observations are inaccurate and unstable even when the GPS/IMU integration technology is used.
A second method of alignment angle estimation involves a process of estimating the alignment angle by the following method after setting the alignment angle by the aforementioned first method.
It is assumed in the following explanation that the inertial attitude sensing system employs angular velocity sensors, for example.
An angular velocity (hereinafter referred to as the GPS angular velocity) ωg1 is calculated from the attitude of the mobile unit obtained by the GPS attitude sensing system while, at the same time, an angular velocity (hereinafter referred to as the IMU angular velocity) ωi1 is determined by the angular velocity sensors. By taking a difference between the GPS angular velocity ωg1 and the IMU angular velocity ωi1, a difference value Δz1 is obtained and an alignment angle θi1 is estimated from the difference value Δz1. The alignment angle θi1 thus calculated is used to correct an IMU angular velocity ωi2 obtained in a succeeding measurement cycle. Then, taking a difference between the IMU angular velocity ωi2 and a GPS angular velocity ωg2 obtained at the same time, a new difference value Δz2 is calculated, and from the difference value Δz2 thus obtained, a new alignment angle θi2 is estimated and used for correcting an IMU angular velocity obtained in a succeeding measurement cycle. This calculation cycle is repeated thereafter such that the alignment angle θi converges to a specific value, whereby a true alignment angle is obtained.
The alignment angle does not converge due to nonlinear property unless the alignment angle calculated as shown above is a small angle of a few degrees. Therefore, the alignment angle needs to be a small angle as an initial condition if the aforementioned method of alignment angle estimation is to be used.
If the GPS antennas are to be installed onboard by a user, for instance, the user must determine a GPS coordinate system (antenna coordinate system) on site and install the GPS antennas at precise positions in such a fashion that the GPS coordinate system substantially matches the IMU coordinate system. From a practical viewpoint, however, it is extremely difficult for the unskilled user to make sure that the GPS antennas are installed with a minor alignment angle between the GPS coordinate system and the IMU coordinate system. Furthermore, if the user can not visually check the locations of inertia sensors from installation sites of the GPS antennas, it is impossible to align the GPS coordinate system with the IMU coordinate system, so that it is absolutely difficult to minimize the alignment angle.